1. Technical Field
The present invention relates to recombinant Anaplasma marginale major surface protein (MSP)1a, related vaccines and methods useful to reduce infections in ticks and affect the biological transmission of the pathogen of the species A. marginale. 
2. Background
Anaplasmosis is a tick-borne disease of cattle caused by Anaplasma marginale (Rickettsiales: Anaplasmataceae). The only known site of development of A. marginale in cattle is within erythrocytes [1]. The number of infected erythrocytes increases logarithmically during infection and removal of infected erythrocytes by phagocytic cells of the reticuloendothelial system often results in development of anemia and icterus without hemoglobinemia and hemoglobinuria [2]. While mechanical transmission of A. marginale occurs when infected blood is transferred from infected to susceptible animals by biting flies or blood-contaminated fomites, biological transmission is effected by feeding ticks. Approximately 20 species of ticks have been incriminated as vectors worldwide [3, 4]. Cattle that recover from acute infection remain persistently infected and develop life-long immunity against clinical disease, but they serve as reservoirs of infection for mechanical and/or biological transmission by ticks.
The development of A. marginale in ticks is complex and coordinated with the tick feeding cycle [5, 6, 8]. In the cycle of A. marginale that was described in male ticks transferred from infected to susceptible hosts, the first site of infection occurs in tick gut cells. After the ticks feed a second time, many other tick tissues become infected, including the salivary glands from where the rickettsiae are transmitted to cattle during feeding. Male ticks become persistently infected with A. marginale and are able to transmit A. marginale to multiple hosts [6, 7, 8].
Major surface protein (MSP)1a is one of six MSPs that have been described on A. marginale derived from bovine erythrocytes [9]. MSP1a forms the MSP1 complex with MSP1b [10, 11]. MSP1a is encoded by a single gene, msp1α, which is conserved during the multiplication of the bacterium in cattle and ticks [12, 13]. This protein is variable in molecular weight among geographic isolates because of varying numbers of tandem 28 or 29 amino acid repeats located in the amino terminal portion of the protein [11, 14, 15]. MSP1a was shown to be an adhesin for bovine erythrocytes and for both native and cultured tick cells using recombinant E. coli expressing MSP1a in microtiter hemagglutination and adhesion recovery assays and by microscopy [16, 17, 18]. Furthermore, MSP1a was shown to effect infection and transmission of A. marginale by Dermacentor spp. ticks [19] and was also shown to be involved in bovine immunity to A. marginale infection [20, 21, 22, 26]. See also U.S. Pat. No. 10/002,636, incorporated herein by reference.
Recently, we demonstrated that infection of A. marginale for cultured tick cells was inhibited by antibodies against recombinant MSP1a [23, 24]. While antisera from cattle naturally infected with A. marginale did not inhibit A. marginale infection, antibodies produced in rabbits and cattle immunized with the recombinant MSP1a effected inhibition of A. marginale infection for the cultured tick cells [24]. This inhibitory effect has also been demonstrated using antibodies against a synthetic MSP1a repeated peptide, and this data provided additional evidence that MSP1a plays a role in adhesion of A. marginale to tick cells [15].
Vaccination is the most efficient and economical method for control of anaplasmosis, and development of effective vaccines has been a priority of the cattle industry worldwide [9]. Infected bovine erythrocytes have been the only source of vaccine antigen until recently when a tick cell culture system was developed for propagation of A. marginale and provides an alternative antigen source. The cell culture-derived A. marginale is currently being tested as antigen for use in vaccine development [20, 22]. See also U.S. Pat. No. 5,869,335, incorporated herein by reference.
Thus far, vaccines using erythrocyte or cell culture-derived antigens have effected reduction of clinical disease but have not prevented infection of cattle [9, 20, 22, 25, 27, 28, 37]. Also, antibodies in cattle immunized with erythrocyte-derived A. marginale have not caused reduction of A. marginale infections in ticks [7].
The desired result of a vaccine for the control of anaplasmosis is to have a protection effect on the multiplication of A. marginale in the bovine host and a blocking effect on the transmission of the pathogen by the tick vector. Existing vaccines and experimental vaccines, however, including formulations using the recombinant MSP1a, the MSP1 complex and partially purified parasites from infected erythrocytes and cultured tick cells (see U.S. Pat. Nos. 5,549,898; 5,869,335 and 10/002,636 incorporated herein by reference) have not demonstrated any effect on the infection of the tick vector by the pathogen. Therefore, it is desirable to develop vaccines against anaplasmosis with protection effect on the multiplication of A. marginale in the bovine host and an effect on the transmission of the pathogen by the tick vector.
A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached figures, wherein there is described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.